1. Field of the Invention
This application relates to hearing aids. The invention, more specifically, relates to hearing aids having means for enhancing speech reproduction. The invention further relates to a method of processing signals in a hearing aid.
A hearing aid is defined as a small, battery-powered device, comprising a microphone, an audio processor and an acoustic output transducer, configured to be worn in or behind the ear by a hearing-impaired person. By fitting the hearing aid according to a prescription calculated from a measurement of a hearing loss of the user, the hearing aid may amplify certain frequency bands in order to compensate the hearing loss in those frequency bands. In order to provide an accurate and flexible means of amplification, most modern hearing aids are of the digital variety. Digital hearing aids incorporate a digital signal processor for processing audio signals from the microphone into electrical signals suitable for driving the acoustic output transducer according to the prescription. In a digital hearing aid, the reproducible frequency range may be conveniently split up into a plurality of frequency bands by a corresponding plurality of digital band-pass filters. This band-split allows the hearing aid to process each frequency band independently with respect to e.g. gain and compression, providing a highly flexible means of processing audio signals.
2. The Prior Art
WO-A1-98/27787 presents a hearing aid with a percentile estimator for determining noise levels and signal levels in an input signal for the hearing aid. A noise level is determined as a 10% percentile level of the input signal, and a signal level is determined as a 90% percentile level of the input signal. It is possible for the signal processor of the hearing aid to make an educated guess about the presence and the level of speech given the difference between the 90% percentile level and the 10% percentile level. In other words, the difference between the 90% percentile and the 10% percentile determines the level of speech. In the following, this method is denoted the percentile difference method. This way of detecting speech works to satisfaction in steady noise or in quiet surroundings, but may not perform adequately in sound environments where the noise varies a lot, e.g. in a cafeteria, at parties, or where background music is present, because the percentile difference method is rather sensitive to modulated noise.
WO-A1-2004/008801 discloses a hearing aid having means for calculating a speech intelligibility index (SII) of an input signal, and means for enhancing a speech signal by optimizing the SII value of the input signal. During use of the hearing aid, the SII value is constantly analyzed and the signal processing is continuously altered in order to keep the SII at an optimal value for the purpose of enhancing speech and reducing noise. The precision of this system is very high, but its adaptation speed is poor due to the complex and involved nature of the calculation of the speech intelligibility index. Whenever the noise level rises, the adaptation speed of the speech intelligibility noise reduction system is approximately 1.8-2 dB/s, and about 17 dB/s whenever the noise level falls, and this adaptation speed may not be sufficient, e.g. in sound environments where modulated noise is present.